Add OpenShift Dockerfiles + Dockerfile Appendix

Description

This MR exists to add two functions to the UBI Runner:

1. The first is to add a Dockerfile Appendix to the command line and allow us to build boutique Dockerfiles.
2. The second is to add Dockerfiles with proper permissions to run without AnyUID on OpenShift.

I've ensured that the readme is updated with the proper commands and documentation to describe how to enable the Dockerfile Appendix. I've also ensured that absent a variable for Dockerfile Appendix that the scripts act as normal and expected. So, it is failsafe as far as Dockerfile Appendix goes.

Related issue

gitlab-org/gitlab-runner#27126 (closed)